The present invention is directed generally to optical transmission systems, devices, and methods that provide for controllably varying characteristics of optical signals passing through the system. More particularly, the invention relates to optical systems, devices, and methods including signal varying devices, such as optical amplifiers, attenuators, and filters that have controllable gain, loss and transparent intensity profiles, and which can include and be responsive to one or more local and remote controllers.
The continued development of digital technology has provided electronic access to vast amounts of information. The increased access to information has fueled an increasing desire to quickly obtain and process the information. This desire has, in turn, driven demand for faster and higher capacity electronic information processing equipment (computers) and transmission networks and systems linking the processing equipment (telephone lines, cable television (CATV) systems, local, wide and metropolitan area networks (LAN, WAN, and MAN)).
In response to this demand, telecommunications companies have turned to optical communication systems to provide substantially larger information bandwidth transmission capacities than traditional electrical communication systems. Early optical transmission systems, known as space division multiplex (SDM) systems, transmitted one information signal using a single wavelength in a waveguide, i.e. fiber optic strand. Time division multiplexing (TDM) multiple information signals onto a single wavelength in a known sequence that can be separated upon receipt has further increased the transmission capacity of optical systems.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing data has further accelerated the demand for higher capacity communications networks. Telecommunications companies have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems. In WDM transmission systems, pluralities of distinct TDM or SDM information signals are carried using electromagnetic waves having different wavelengths. The pluralities of information carrying wavelengths are combined into a multiple wavelength signal, which is transmitted in a single waveguide. In this manner, WDM systems can increase the transmission capacity of existing SDM/TDM systems by a factor equal to the number of wavelengths used in the WDM system.
Optical WDM systems were not initially deployed, in part, because high cost electrical signal regeneration/amplification equipment was required for each optical wavelength throughout the system. However, the development of the erbium doped fiber optical amplifier (EDFA) eliminated the need for electrical signal regeneration/amplification equipment and the associated costs in many systems, thereby making WDM a cost effective means to increase network capacity.
Erbium doped fiber amplifiers (xe2x80x9cEDFAsxe2x80x9d) can theoretically be used to amplify signals in an amplification wavelength range spanning from approximately 1500 nm to 1600 nm. However, EDFAs do not equally amplify each optical signal wavelength within the range. The differences in amplification can result in attenuation of some signals and/or signal loss or distortion because of highly amplified noise. Thus, the performance of EDFAs in a transmission system varies depending upon the number of wavelengths and the wavelengths used in the system.
Judicious selection of the wavelengths and amplifier powers used in a system can minimize EDFA variations (gain non-uniformities). For example, many WDM systems currently restrict the wavelengths used in the system to between 1540 nm and 1560 nm, a range in which EDFAs comparably amplify optical signals. As might be expected, restricting system designs to only those wavelengths that are comparably amplified by EDFAs severely limits the number of wavelengths and the information transmission capacity of WDM systems.
The number of wavelengths in the system can be increased to some extent, if only a small number of amplifiers are used in the system. The small number of amplifiers allows wavelengths having differing EDFA amplification characteristics to be used, because the cumulative amplifier variations do not swamp out lowly amplified signals.
In addition to the wavelength dependence, EDFA performance is also a function of the amplification power supplied to the EDFA. Thus, EDFAs generally must be operated with a limited power range to minimize amplification variations in the system. The amplifier power limitations, in turn, increase the number of amplifiers in a system by limiting the allowable distance between EDFAs, i.e., the span length.
In discussing the signal intensity variation of EDFAs and other devices, the uniformity of gain or loss profiles over a wavelength range is generally referred to as the flatness of the profile. A perfectly flat profile is a gain, loss, or transparency profile that has a constant value over the wavelength range of interest.
WDM system constraints imposed by EDFA wavelength variations have focused attention on providing EDFA configurations that compensate for the variations and provide more uniform gain for a larger band of wavelengths and over a greater power range. Various EDFA configurations have been proposed to minimize amplifier gain variations. For example, see U.S. Pat. Nos. 5,406,411, 5,541,766, 5,557,442, 5,636,301, and 5,696,615; Sugaya et al., Optical Amplifiers and Their Applications, Technical Digest OSA 1995 v. 18, pp. 158-161/FC3-1; Jacobovitz-Veselka et al., Optical Amplifiers and Their Applications, Technical Digest OSA 1995 v. 18, pp. 162-165/FC3-1; Park et al., Electronics Letters, Mar. 5, 1998, Vol. 34, No. 5, Online No. 19980346; and, Dung et al., Electronics Letters, 19 Mar. 1998, v. 34, n. 6, Online No. 19980446.
Other amplifier configurations have used EDFAs in combination with a Raman amplifier to statically vary the gain profile of an EDFA. For example, see Masuda et al., OSA 1997, pp. 40-3/MC3-1, Masuda et al., Electronics Letters, v34, n13, Online No. 19980935 (Jun. 25, 1998), and U.S. Pat. No. 5,083,874 issued to Aida et al. It has also been proposed to eliminate EDFAs and use amplifier configurations that employ only Raman amplifiers. However, the all-Raman configurations to date have not greatly improved the amplifiers gain flatness profile and may still require gain equalization to flatten the gain profile as discussed by Rottwitt et al., xe2x80x9cA 92 nm Bandwidth Raman Amplifierxe2x80x9d, OFC 98, p. 72/CAT-1.
The above referenced gain flattened configurations are generally statically configured to have a wavelength range defined by a 3 dB variation (xcx9ca factor of 2) in the gain profile and having a xc2x11 dB variation between wavelengths. The gain flattened amplifiers provide some improvement over conventional EDFAs in the number of amplifiers, amplifier power ranges, and span lengths before the signal must be regenerated. The gain flattened optical amplifiers nonetheless introduce excess amplifier noise and gain nonuniformities that limit the number of optical amplifiers that can be used in a WDM system prior to signal regeneration.
Gain flattening in optical amplifier configurations is generally performed using filters and/or attenuators to decrease the signal intensity of the wavelengths to a specified value. For example, in many embodiments, the optical signals are amplified to an intensity higher than the amplifier output value and the filters and attenuators are used to flatten the gain profile by decreasing the optical signal intensity. These methods tend to increase the noise in the signal with a corresponding decrease in the output power of the device.
Optical filters and attenuators can be separate optical devices added to the system or all-fiber devices, such as Bragg grating filters and all-fiber attenuators as discussed in U.S. Pat. Nos. 4,728,170, 5,095,519, 5,633,974, 5,651,085, and 5,694,512. The filters and attenuators can be variable or fixed depending upon the configuration. The amplifier, filters, and attenuators are configured statically to flatten the gain profile.
As the demand for transmission capacity continues to increase, there is an increasing need for systems that cover longer distances and provide for an increasing number of information carrying wavelengths/channels. Thus far, it has proven difficult to balance the non-linear gain of EDFA configurations with selective wavelength filtering and attenuation to provide gain flattened amplifier configurations that meet this need.
Accordingly, there is a need for signal varying devices generally, and optical amplifiers and attenuators particularly, that provide increased control over the intensity profile of optical signal in the optical systems. The improved signal varying devices will provide for higher capacity, more versatile, longer distance communication systems.
The apparatuses and methods of the present invention address the above difficulties with prior art optical devices and systems. An optical system of the present invention includes a plurality of optical processing nodes in optical communication via at least one signal varying device. The signal varying device includes an optical fiber suitable for facilitating Raman scattering/gain in a signal wavelength range and a pump power source for providing pump power in a plurality of pump wavelengths. The pump source provides sufficient pump power in each pump wavelength to stimulate Raman scattering/gain in the optical fiber within the signal wavelength range.
The signal varying device may be embodied as a distributed device that employs a portion or all of an optical transmission fiber extending between two optical nodes, such as between an optical transmitter and an optical receiver. The signal varying device may also be embodied as a lumped or concentrated device that is placed in the optical transmission fiber at discrete locations between the optical nodes.
The pump wavelengths are selected such that the combined Raman gain resulting from the pump power supplied by each pump wavelength produces a desired signal variation profile in the signal wavelength range. In addition, the pump power supplied by at least one of the pump wavelengths may be dynamically varied to produce a controlled signal intensity variation profile over the signal wavelength range in the optical fiber. In an embodiment, four pump wavelengths spaced in 10-30 nm intervals may be used to provide intensity gain and flatness control to over 30 nm to within xc2x10.2 dB.
Also in an embodiment, erbium doped fiber is included in the signal varying device to provide a multiple stage signal varying device. The erbium doped fiber and the multiple wavelength controlled Raman portion of the signal varying device may be operated in conjunction to impart a desired intensity profile to the optical signal.
The design and length of the optical fiber used in conjunction with the pump source may be tailored to provide flexibility in operation of the system. For example, a concentrated, or lumped, high gain signal varying device may be provided using a small core fiber, such as dispersion compensated or dispersion shifted fiber. The lumped device further provides for a greater range over which the signal varying device may be used as an attenuator because of its higher localized loss.
Multistage concentrated and/or distributed Raman signal varying devices may also be employed to further tailor the profile using either separate or common pump sources. For example, a first concentrated Raman stage may employ small core fiber to provide for efficient Raman amplification of the signal wavelengths. A second concentrated Raman stage may employ a larger core fiber to further amplify the signal power, while lessening the extent of non-linear interactions amongst the signal wavelengths that may occur in a single stage with smaller core fibers. The second concentrated Raman stage may also employ fiber having low loss in the 1400-1520 nm range to allow for more efficient Raman pumping of the multiple stages using a common source. In addition, the first and second Raman stages may use fibers that have different chromatic dispersion characteristics to further reduce the extent of non-linear interaction between the signal wavelengths.
Distributed signal varying devices may be provided by employing the optical transmission fiber spanning between the optical nodes to control the signal variation profile occurring in the transmission fiber. Also, different optical fiber types, including doped fibers, may be used in various portions to replace existing transmission fiber to provide for different distributed signal varying profiles. The concentrated and distributed Raman signal varying devices may be used alone or in combination to statically or dynamically impart desired signal varying profile characteristics to the system.
In an embodiment, a distributed Raman amplifier may be employed with one or more first pump sources propagating pump power in the transmission fiber to amplify counter-propagating signal wavelengths to provide a first signal varying profile. A concentrated Raman signal varying device may be placed in series with the distributed Raman amplifier employing one or more second pump sources to provide a second signal varying profile. The first and second signal varying profiles acting to produce a desired overall signal varying profile. Additionally, an EDFA may be employed to contribute a third signal varying profile to the overall signal varying profile.
A distributed Raman amplifier may also be used to provide pump power to one or more remotely located concentrated or distributed Raman amplifiers and/or doped amplifying fibers. For example, the pump sources may be selected to produce a first signal varying profile in the distributed Raman amplifier and a second signal varying profile in the remotely located erbium doped fiber. The pump power and/or the wavelength of the pump power sources may be varied to control to individual and overall signal varying profiles. Pump power may also be supplied to remotely located signal varying devices using one or more separate fibers. Such fibers may be pure SiO2 to minimize loss and nonlinear conversion of the pump light.
Additional gain and gain profile control in Raman amplifier stages may be produced by including one or more pumps at lower Raman wavelengths that serve to provide additional pump power to the higher Raman pump wavelengths. The pump source may employ numerous configurations to decrease the extent of interference, i.e., cross-talk, that occurs between the Raman pump wavelengths, as well as the signal wavelength.
Thus, the devices and methods of the present invention provide for control of the signal intensity over a range of wavelengths in optical transmission systems. Accordingly, the present invention addresses the aforementioned problems and provides signal varying devices, methods, and optical systems that provide increased control over optical signal characteristics in the system. These advantages and others will become apparent from the following detailed description.